Assessment of the radiometric quality of TerraSAR-X and COSMO-SkyMed SAR sensors

The retrieval of biophysical parameters of soil and vegetation using Synthetic Aperture Radar (SAR) images requires radiometrically well-calibrated sensors. In this paper, the radiometric quality of TerraSAR-X (TSX) and COSMO-SkyMed (CSK) constellation (four satellites: CSK1, CSK2, CSK3, and CSK4) was assessed. The analysis of the X-band SAR signal on a forest stand showed a stable signal (with variation lower than 1 dB) over time for each of the studied sensors but a significant difference was observed between the different X-band SAR sensors. Differences between SAR’s were higher in HH than in HV polarization. TSX and CSK4 showed very similar backscatter signals, with differences in the signal levels of 0.7 dB in HH and 1.2 dB in HV. The CSK3 signal was observed to be lower than those from both TSX and CSK4 of about 1.8 dB and 1.1 dB in HH against 3.1 dB and 1.9 dB in HV, respectively. Moreover, CSK2 and CSK1 which showed similar backscatter signals (within 0.6 dB in HH and 1.4 dB in HV) had signal levels lower than that from TSX of 2.5-3 dB in HH and 4.5-5 dB in HV (for about 29° incidence angle). These results suggest that, if we assume that the studied forest is a stable zone, the calibration constants of some of the studied SAR should be refined. Without additional correction, it is not possible to use jointly the different current X-band SAR for estimating the biophysical parameters of soil or vegetation for example.